Let's see if I can remember the C3 ratins as it's been awhile -
It is very important that you know how to lunge correctly, the Pony Club way, in a proper lunging cavesson. You also need to remember to have gloves and a helmet while lunging. You need to *practice* this skill with your horse, because they are fairly strict about how you handle the extra loops of line and your whip and they want to see you control and influence your horse's way of going, not just have them loaf along at the end of the line.
Also, it's more important to be able to give the examiner an accurate analysis of your ride than it is to have a perfect ride. In other words, if you miss a distance to a fence, and tell the examiner that you lost a little pace coming up the hill and failed to correct for it and that caused a deep, weak spot, you'll actually do better than the candidate on a push button horse who nails the distance but can't provide any analysis. So I would watch a lot of You Tube video and education video, and really try to develop your eye and analysis. Ask an instructor if you can stand in the ring during a lesson and give her your version of what happened and have her correct and critique your analysis.
Those were the two big stumbing blocks that I remember for C3, if I remember more, I'll post more.